In the production of gas from subterranean reservoirs, associated liquids, such as oil and water are commonly coproduced. Such liquids are often undesirable, and the rate of the liquid production often increases compared to the amount of gas production. For example, water-coning of a gas well is a serious problem.
Prior efforts have attempted to selectively shut off water production from oil or gas wells. SPE paper No. 14822, by D. D. Dunlap et al. entitled "Method For Improving Hydrocarbon/Water Ratios in Producing Wells", describes the use of an amphoteric polymer which is injected into the productive formation to absorb and preferentially block the water. The idea behind the use of aqueous based polymer gels is to inject a low viscosity polymer solution into the formation that also contains a crosslinking agent. After a suitable shut-in period, the polymer will react and form a stiff crosslinked gel that will decrease the formation permeability.
A major disadvantage is the relatively high cost of both the chemicals and the process. Also, the chemical bonds in polymer gels are prone to thermal degradation after several months in a formation.
U.S. Pat. No. 4,428,424 discloses a method of preferentially blocking water by combining saltwater with a silicate and an acid to produce a silicate that blocks produced water but allows oil to flow. This treatment is also very costly.
Several U.S. Pat. Nos. 4,869,621; 4,844,164; 4,287,951; 4,301,867; 3,342,262; 4,773,483; 4,745,976 suggest using inorganic precipitates such as scale as a means to plug off production from productive geologic formations. Generally, these methods require sequentially injecting incompatible fluids, such that in-situ mixing occurs to trigger precipitation. None of these methods use a scale inhibitors compound to act as a time control for the process
Scale formation can occur as a result of mixing of incompatible waters in the well, i.e., waters which when mixed produce precipitates, or as a result of temperature and pressure changes and the like in the produced waters during production. Generally, incompatible waters are formed as a consequence of waterflooding, for example, as injected sea water mixes with formation water in the borehole during water breakthrough. The more common concern is scales deposited because of changes in supersaturation of solubility of minerals in the formation or produced waters caused by pressure and temperature changes, or changes in other physical and chemical environments such as gas compositions, ratio of gas/oil/water. Precipitation frequently encountered as scale include calcium carbonate, calcium sulfate, barium sulfate, magnesium carbonate, magnesium sulfate, and strontium sulfate. The deposition of scale is a very complex crystalline process initiated by a supersaturation-induced nucleation of a precipitate of the mineral, scale ions contact these nuclei and the crystal grows in a certain crystalline pattern. The adherence of these mineral crystals unto the formation matrix, perforation, wellbore, tubings and equipment is a not well-understood process but once initiated, appears to be spontaneous as seen by the increasing thickness of the scale deposit and the steady decline in productivity.
U.S. Pat. Nos. 4,002,204; 3,730,272; 3,682,245; and 3,747,677 disclose methods of incorporating time control over deliberate precipitation to reduce fluid production, however none use a scale inhibitor.
U.S. Pat. No. 4,947,934 discloses a method of inhibiting the formation of scale in a well by injecting an acidic aqueous solution of a polyvalent cation and a polyacrylate scale inhibitor. As the acid is consumed by the reservoir rock, the pH of the solution is sufficiently raised to cause controlled precipitation. This method avoids premature plugging of the formation and extends the life of the scale inhibitor treatment. There is no discussion of preferentially blocking the flow of liquid in favor of gas in the well. The use of such scale inhibitors is well known in the art to protect scales from forming in the perforations or near well bore areas or other equipment.
U.S. Pat. No. 5,002,126 describes a method of inhibiting scale in a non-carbonate reservoir by injecting a soluble surfactant metal salt containing a terminal metal ion which is absorbed on the rock surface, and thereafter injecting scale inhibitor. The result is that scale inhibition is provided over an extended period of time. There is no discussion of any effect on gas vs. liquid production form the reservoir.
None of the prior art discloses the injection of scale inducing brines with a scale inhibitor to preferentially reduce the production of liquids from a reservoir. There is therefore a need for such a method that can be performed quickly and economically.